题干

在2011年5月15日进行的国际田联钻石联赛上海站中,首次尝试七步上栏的刘翔以13秒07创项目赛季最好成绩夺冠。他采用蹲踞式起跑,在发令枪响后,右脚迅速蹬离起跑器,在向前加速的同时提升身体重心。如下图所示,假设刘翔的质量为m,在起跑时前进的距离s内,重心升高量为h,获得的速度为v,克服阻力做功为W,则在此过程中                                                                              (    )

A:刘翔的机械能增加了

B:刘翔的重力做功为

C:刘翔自身做功为

D:刘翔自身做功为

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答案(点此获取答案解析)

D

同类题4

                                                                                 The Girl From the Street
    The workers who brought the girl to the orphanage(孤儿院)knew little about her. The streets where they found her had been her home for many years. Her parents were unknown. They left her long ago. At the orphanage, the girl, like all the children there, was taught to read and write. While she was studying at the orphanage, she learned something else—to be independent(独立). At twenty-one, she left the orphanage and began working as a secretary.(秘书)And then, in 1975, while she was still working as an ordinary secretary, something special happened. She entered the Miss Hong Kong competition and won it. This was the turning point in her life. Now her name, Mary Cheung, was known to everybody.
    Mary entered the competition because she wanted to show that orphanage girls could be something. Winning the competition gave her the chance to start a new life. This led her first into television and then into business as a manager. When she was working as a manager, she had trouble with her reports. “My English just wasn't good enough.” She says. Luckily, she had a boyfriend (who later became her husband) to help her.
    Mary studied management(管理)at Hong Kong and graduated in 1980, She started her own business in 1985. But she did not stop developing herself. She then studied at the University of Hong Kong. Since 1987, she had spent a lot of time on photography. She has held several exhibitions of her work in many places---China, New Zealand and Paris. She still found time, however, to work on TV, write for newspapers and bring up her family.
    The girl from the street has come a long way, but her journey is not finished yet.